Shapiro reaction mechanism

The reaction mechanism has been confirmed by trapping of intermediates 13, 14 and 15. Because of the fact that neither a carbene nor a carbenium ion species is involved, generally good yields of non-rearranged alkenes 2 are obtained. Together with the easy preparation and use of tosylhydrazones, this explains well the importance of the Shapiro reaction as a synthetic method. 

What is the mechanism of the cyclopropanation step g How does this step differ in the number of equivalents of base required with respect to a Shapiro reaction ... 

The reaction of tosylhydrazones with sodium in ethylene glycol to give alkenes had been observed before (Bamford-Stevens reaction) other bases, e.g. NaOMe, alkali metal hydrides and NaNH2 were also used. However, in these cases side reactions occur and, in contrast to the Shapiro reaction, the more highly substituted alkene is predominantly formed. Two mechanisms are discussed for these reactions a mechanism via a carbenium ion, which usually takes place in protic solvents, and a carbene mechanism in aprotic solvents (Scheme 28). In both cases diazo compounds are intermediates, which can sometimes be isolated. ... 

The mechanism of the Shapiro reaction is believed to involve initial deprotonation of the NH proton from tosylhydrazone 5 to generate 6, which undergoes a second deprotonation adjacent to the hydrazone group to afford dianion 7. Elimination of lithium p-toluene-... 

The Bamford-Stevens synthesis is related to the Shapiro reaction (Shapiro and Heath, 1967 reviews Shapiro, 1976 Adlington and Barrett, 1983), in which a 4-toluenesulfonyl hydrazone of an aldehyde or a ketone is treated with at least two equivalents of a very strong base, usually, methyllithium (see Organic Syntheses examples of Chamberlin et al., 1983, and Shapiro et al., 1988). The Shapiro reaction leads to an olefin by a hydrogen shift. The mechanism has been proposed by Casanova and Waegell (1975) as given in (2-34). This mechanism involves a diazenide anion 2.81 as intermediate. 

S. A. Rice My answer to Prof. Manz is that, as I indicated in my presentation, both the Brumer-Shapiro and the Tannor-Rice control schemes have been verified experimentally. To date, control of the branching ratio in a chemical reaction, or of any other process, by use of temporally and spectrally shaped laser fields has not been experimentally demonstrated. However, since all of the control schemes are based on the fundamental principles of quantum mechanics, it would be very strange (and disturbing) if they were not to be verified. This statement is not intended either to demean the experimental difficulties that must be overcome before any verification can be achieved or to imply that verification is unnecessary. Even though the principles of the several proposed control schemes are not in question, the implementation of the analysis of any particular case involves approximations, for example, the neglect of the influence of some states of the molecule on the reaction. Moreover, for lack of sufficient information, our understanding of the robustness of the proposed control schemes to the inevitable uncertainties introduced by, for example, fluctuations in the laser field, is very limited. Certainly, experimental verification of the various control schemes in a variety of cases will be very valuable. 

Another strategy for optical control of chemical reaction dynamics has been investigated by Brumer and Shapiro. The simplest version of their scheme relies on quantum mechanical interference between one and three photon absorption pathways to a given final continuum state [16]. Thus a fixed relative optical phase between continuous wave light sources of different colors must be maintained. Some essential features of Brumer and Shapiro s proposed method have been implemented in recent experimental work [17]. 

Detailed studies have been reported of the kinetics of the methyl transfer between S-adenosylmethionine and homocysteine, catalyzed by a 70-fold purified enzyme from yeast (Shapiro et al., 1965). The evidence supported a mechanism in which homocysteine is the first substrate bound to the enzyme and methionine is the last product released. This study demonstrated also that the reaction is inhibited by both methionine and S-adenosylhomocysteine (Shapiro et al., 1965). S-Adenosylhomo-cysteine inhibits the activity of a number of other methyltransferases (Zappia et al., 1969b), and such product inhibition has been suggested as a general regulatory mechanism for this class of reactions (Zappia et al., 1969b). This postulate gains additional plausibility because, at least in liver, the concentrations of S-adenosylhomocysteine and S-adenosylmethionine are of the same order of magnitude (Salvatore et al., 1968). 

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